A well-known trend in the semiconductor industry is toward smaller, higher speed devices. In particular, both the lateral dimensions and the depths of features in semiconductor devices are decreasing. State of the art semiconductor devices require junction depths less than 1,000 angstroms and may eventually require junction depths on the order of 200 angstroms or less.
One critical element in the trend toward increased miniaturization is the gate structure of MOS transistors. A dielectric layer overlays the channel, and the gate electrode is disposed on the dielectric layer. Prior art devices have typically utilized a silicon dioxide gate dielectric.
As device dimensions decrease and operating speeds increase, the thickness of the gate dielectric must be reduced. However, below a thickness of about 1.5 nanometers, processing difficulties are encountered and leakage current may be unacceptable. One proposed solution to this problem involves the formation of dielectric layers having higher dielectric constants. For example, silicon nitride and silicon oxynitride dielectric layers have been utilized, and zirconium and hafnium oxide dielectric layers have been proposed. See for example, Hiroshi Iwai et al, “ULSI Process Integration for 2005 and beyond”, Electrochemical Society Proceedings Volume 2001-2, pages 3–33 and Howard R. Huff et al, “The Gate Stack/Shallow Junction Challenge for Sub-100 nm Technology Generations”, Electrochemical Society Proceedings Volume 2001-2, pages 223–241.
Techniques for forming silicon nitride or silicon oxynitride films have included chemical vapor deposition (CVD), remote plasma enhanced chemical vapor deposition (RPECVD), low pressure rapid thermal chemical vapor deposition (RTCVD), jet vapor deposition (JVD), in situ steam generation (ISSG) with remote plasma nitridation (RPN), and reoxidation of silicon nitride in a vertical high pressure (PHP) furnace. In each of these techniques, heating is required to diffuse the nitrogen to the desired depth and to promote a chemical reaction between the nitrogen and the silicon dioxide. However, the required heating may cause diffusion of the nitrogen beyond the thin dielectric layer and may cause undesired diffusion of other doped materials in the device being fabricated.
Accordingly, there is a need for improved methods for forming ultra thin dielectric layers and metallic layers.